This invention relates to a radial tire for passenger cars, which has an improved driving stability without sacrificing the riding comfort.
Radial tires for passenger cars generally have a carcass layer made of an organic fiber such as nylon, polyester, rayon or aromatic polyamide and a belt layer comprising steel cords crossing one another. By virtue of the insertion of steel cords, radial tires have improved wear resistance, high speed performance and driving stability over conventional bias tires as well as rolling resistance smaller than that of the conventional bias tires.
In general, a typical structure of the bead portion of radial tires includes (1) one wherein a bead filler having a high hardness is interposed between the body of the carcass layer and the turnup portion of the carcass layer in order to attain lateral stiffness through the "sandwich effect"; and (2) one wherein the turnup portion of the carcass layer is provided along the body of the carcass layer and an upper bead filler having a Shore hardness over 90 is provided outside the turnup portion, thereby improving the riding comfort without spoiling the driving stability, as described in Japanese Utility Model Publication No. 963/1984.
In the structure of the bead portion of type (1), a bead filler having a high hardness is interposed between the body of the carcass layer having an enhanced apparent stiffness due to the internal pressure and the turnup portion. This greatly enhances the bending stiffness, which in turn enhances the lateral stiffness of the tire associated with an improvement in the driving stability. However, this also unfavorably enhances the longitudinal stiffness of the tire associated with the riding comfort. As described in Japanese Patent Laid-Open No. 11443/1984, the insertion of a chafer into the bead portion improves the lateral stiffness. However, this additionally reinforces the portion between the body of the carcass layer having improved apparent stiffness by virtue of initial tension due to internal pressure and the turnup portion, which leads to lowering in the riding comfort.
In the structure of the bead portion of type (2), the lateral stiffness of the tire is attained through the compression stiffness of the upper bead filler without utilization of the "sandwich effect" between the body of the carcass layer and the turnup portion. Therefore, the longitudinal stiffness in this structure is lower than that of the structure of the bead portion of type (1), which makes it possible to improve the riding comfort. However, since the lateral stiffness of the tire inevitably lowers, a significant lack of the lateral stiffness is observed when the tires are mounted on a high performance passenger car. Therefore, such a structure of the bead portion is unsatisfactory in the driving stability.